Whether on his own or as part of the long-running rock-soul soap opera that is the J. Geils Band, singer-songwriter-harmonicat Peter Wolf is legendary for his roots. Not tonsorial roots (though he does have good rock-guy hair, especially for a 70-year-old), but his formation in the firmament of classic, ragged R&B, blues and - from the sound of his new album, A Cure for Loneliness - bluegrass.

Wolf will be in Philadelphia Saturday to play WXPN-FM's Big Night Out fund-raiser at World Cafe Live.

"I grew up with it all," Wolf says about a childhood lived in New York City before heading to Boston and, eventually, rock stardom. "By the age of 10, I was fortunate enough to have seen the Alan Freed show and his great array of rock-and-roll talent on one stage." Wolf name-checks rock's founders - Chuck Berry, Little Richard, Buddy Holly, Screamin' Jay Hawkins, Jerry Lee Lewis - before discussing his parents' love of folk (Woody Guthrie, the Weavers) and his own youthful "sneaking into Birdland" to witness Dinah Washington, Art Blakey and the Jazz Messengers, and Thelonious Monk.

"All of these different threads of music were a daily part of my upbringing. Music was just music. Plus, it was the golden age of show tunes - Rodgers and Hammerstein and Irving Berlin and early Sondheim [lyrics] with West Side Story.

"This was a New York where you could see Miles Davis at the Blue Note and James Brown at the Apollo all in one day, especially since the latter was, like, 10 blocks from my high school. I went there every day for three years - that was where I learned all of my tricks of the trade."

For good measure, Wolf throws in countless hangs with Jerry Blavat in Philadelphia, where the Geator brought the rocker to "bars where you had to go behind another bar to the main bar where the real action was," he says with a chuckle.

For all that raw R&B and rock that Wolf grew up with - and played in ensembles such as the proto-punk Hallucinations and, starting in 1967, the J. Geils Band - Wolf's biggest hits came in the '80s with the slick, new-wave pop sounds of "Centerfold," "Freeze Frame" (both with J. Geils) and his own Lights Out.

Wolf says these sleeker sounds represented the band's organic growth, rather than a commercial ploy. "The landscape changed with MTV, the technical stuff of synthesizers and the way radio worked at that time," he says. "We adapt."

What is pertinent about that slick moment in time, where A Cure for Loneliness is concerned, is that Wolf chose, on his newest album, to turn that era's slam-banging "Love Stinks" into a bluegrass jam.

"There was this station you could hear in the Bronx - WDWW, from West Virginia - that almost always played bluegrass," he says. "It really stuck with me, as did meeting Bill Monroe. This band of mine would play acoustic every show and throw in Monroe numbers. One night, we just started kidding around with 'Love Stinks' and just happened to be running tape."

Much of Cure for Loneliness "just happened." The album is a mix of rough-edged live recordings and studio sessions. There's a cover of a Moe Bandy classic, "It Was Always So Easy (to Find an Unhappy Woman)," and Wolf originals penned with the late soul man Don Covay and lyricist Will Jennings, whom Wolf calls a "master" and a "maestro," respectively. While the live tracks on Loneliness were "happy accidents," Wolf says, Jennings and Covay pushed him to be better.

"There is no one constant in my career - no tell - save for trying to do great work."

Peter Wolf and the Midnight Travelers play Big Night Out WXPN fund-raiser at 7 p.m. Saturday at World Cafe Live, 3025 Walnut St. $100. 800-565-9976, xpn.org.